I kind of wish they released all invites at once like some other schools. That'd help me plan my R2 apps better.
In any case, don't start getting too nervous until the end of November-ish. Thanksgiving in the US is the last week on November, so I could see some invites going out as late as the first week of December. Darden always says they can invite to interview up until the decision deadline, but I would wager that the vast majority of invitations (close to or actual 100%) are sent at least two weeks before decisions go out.